1. Field of the Invention
The present invention relates to an optical recording medium, which is capable of performing recording and readout using short-wavelength laser beam. More particularly, it relates to a write-once type optical recording medium having excellent sensitivity and reliability according to the DVD-ROM standard, which is capable of high-density recording and readout of information by irradiation of a laser beam with a decreased beam diameter using an organic dye thin film whose light reflectance changes with short-wavelength laser beam.
2. Description of the Related Art
Optical recording mediums capable of recording have come into wide use. The optical recording medium has a feature that the recording medium does not cause wear deterioration because a writing (recording) or reading (readout) head is of a non-contact type. Furthermore, the optical recording medium has attracted much attention and has been developed as a large-capacity information carrier because the writing density can be extremely increased by decreasing a spot size of optical beam.
The recording and readout of the optical recording medium are performed as follows. For example, the recording can be performed by condensing laser beam in a recording layer, converting said laser beam into heat energy and changing the properties of the recording film by melting, decomposition, removing, etc. (to form a deformed layer pit or recording it 6 in FIGS. 1 to 3 below), whereas the readout can be performed by a change in amount of reflected light as compared with the unrecorded portion.
As the recording layer, a film of a chalcogenide metal represented by a tellurium alloy was put into practice at first. However, the chalcogenide metal film is disadvantageous in that it is harmful to humans, it formation involves a dry method which suffers high production costs and it is difficult therewith to meet an increasing demand for higher density recording. Therefore, proposals and reports about a medium using a recording layer containing an organic dye as a main component have been increased. This is because although the recording medium composed mainly of an organic dye has a lower reflectivity than metal, it has various excellent properties: (1) a recording layer can be made by forming a thin film using a low-cost wet method represented by a spin-coating method, although the reflectance is lower than that of the metal; (2) it is not corroded because of its excellent oxidation resistance; (3) recording pits with sharp shapes can be formed since it has a lower heat conductivity than metals and, hence more localized heating is possible.
On the structure of media, there have been made various proposals, e.g., a so-called air sandwiched structure obtained by providing an air layer on a recording layer of a generally used dye film and an optical recording medium capable of obtaining a readout signal corresponding to the CD standards. These structures are described in Japanese Patent Application Publication No. 3-759343, Japanese Patent Application Laid-open No. 2-87341, Japanese Patent Application Laid-open No. 5-67352 and Nikkei Electronics No. 469, page 107, Jan. 23, 1989.
In the optical recording disc in accordance with the CD standards, for example, there are known those discs obtained by forming a light absorption layer of an organic dye on a light transmitting resin substrate, forming a light reflection layer represented by Au directly or via a hard layer thereon and forming a resin protective layer on this light reflection layer. The light reflection layer is formed because a high reflectance of 65% or more can not be obtained by using only the organic dye film.
Irradiation of a laser beam to this disc allows the organic dye layer to absorb light so that it is molten or decomposed and, at the same time, the substrate is softened and the dye is mixed with the substrate at the interface. Finally, there is formed a recorded pit whose interface is deformed. In the deformed layer pit thus formed, the light reflectance varies with the light phase difference and reading can be performed similarly to CD.
As the organic dye layer of this optical disc, there are known, for example, squarilium dyes (Japanese Patent Application Laid-open Nos. 56-46221, 63-218398, 1-178494, 5-139047 and 7-44904), naphthoquinone dyes (Japanese Patent Application Laid-open Nos. 61-290092, 62-432, 63-168201 and 5-139047), azo dyes (Japanese Patent Application Laid-open Nos. 7-161069, 7-251567 and 8-99467), phthalocyanine dyes (Japanese Patent Application Laid-open Nos. 57-82094, 57-82095, 7-156550, 7-16068 and 7-52544) and cyanine dyes represented by the following general formula (III) (Japanese Patent Application Laid-open Nos. 59-24692, 2-87341, 6-320869, 6-338059, 6-199045, 7-262611, 62-201288 and Japanese Patent Application Publication 7-4981). ##STR1##
Wherein R.sup.15 and R.sup.16 independently represent an alkyl group, an aryl group or an alkoxyl group; Y.sup.3 and Y.sup.4 independently represent a halogen atom, a hydrogen atom, an alkyl group, an alkoxyl group, an aryl group, an alkoxysulfonyl group, a sulfonylalkyl group or a cyano group; Q.sup.1 and Q.sup.2 independently represent a sulfur atom, an oxygen atom, a selenium atom or an ethylene group; Z is an anion; and p and q are independently an integer.
Among the above dyes, cyanine dyes are mainly used in view of the sensitivity, C/N ratio, thermal properties, ease of converting into a thin film, etc. Particularly, the cyanine dyes are generally dyes having absorption and reflectance at a semiconductor laser wavelength of about 780 to 830 nm are used so that the resulting recording medium can be in accordance with the CD standards. It is known there is the length of .pi. conjugated system as one of factors that determine the absorption wavelength of a molecule. Those cyanine dyes with a methylene chain in the central part of a cyanine dye molecule having absorption near 780 to 830 nm are generally those cyanine dyes which are represented by general formula (III) wherein the number, p, of methylene chains in the center of the molecular structure is generally 2 (i.e., p=2 in the general formula (III)). However, the cyanine dyes having two methylene chains have a fatal problem that they cannot be applied to short-wavelength laser. Furthermore, it is considered that the cyanine dyes represented by the general formula (III) have problems about readout deterioration, long-term stability of the film, C/N ratio, jitter component, etc. Reasons for the above are (i) that heat accumulation arises in the recording layer by a long-term exposure to reading light and the dye itself is deteriorated and discolored, (ii) that the reading light is absorbed at the interface of the recording layer where the deformed layer portion and non-recorded portion should be discriminated one from the other so that the melting or heat deformation arises slowly, and in addition distortion of the contour of the deformed layer pit arises simultaneously due to the similar heat accumulation, (iii) that when it is excited, the dye is oxidatively deteriorated (discolored) due to singlet oxygen which is generated due to transfer of energy from the dye to oxygen in the atmosphere, (iv) that as the phenomenon of deterioration after a long-term storage, there occur variation in the transmittance of a cyanine dye and generation of noises as a result of photo-deterioration of the dye film by natural light and read-out light as well as oxidation of the dye by oxygen or moisture and association or agglomeration of the dye molecules. To solve these problems, various proposals have hitherto been made (cf., e.g., Japanese Patent Application Laid-open Nos. 62-201288, 62-201289, 57-66541, 59-124894, 59-203247, 57-11090, 60-44389, 60-71296, 63-1594, 5-38879, and 7-262611, and Japanese Patent Application Publication No. 7-4981). However, these problems are still to be sufficiently solved.
On the other hand, there has been made development of an optical disc having high recording density in accordance with the DVD-ROM standards as a high density optical medium. There have been proposed a system in which use is made of a semiconductor laser with a wavelength of about 600 to 680 nm, that is shorter than the wavelength used for currently prevailing CDs, and a decreased beam spot diameter. In order to develop dye materials for DVD-ROMS, there have been made various attempts to improve the cyanine dyes represented by general formula (III) for CDs and proposals for shorter wavelength cyanine dyes (For example, (Japanese Patent Application Laid-open Nos. 6-199045, 7-186530, 8-306074, 5-38879, 6-40162, etc.).
However, various problems are still to be solved. For example, the recording layer does not have a sensitivity nor film stability corresponding to a desired wavelength so that when a high density medium is fabricated, there occur thermal interference between adjacent pits and other undesirable phenomena, which cause decreases in S/N ratio and C/N ratio and an increase in jitter component in a readout signal.